1. Field of the Invention
The present invention relates to a voice coding apparatus and, more particularly, to a voice coding apparatus for carrying out a function of speech coding at a CDMA (Code Division Multiple Access) control station or a base station in PCS (Personal Communication System) service.
2. Information Disclosure Statement
In a cellular mobile communication system of the CDMA method, one mobile terminal can simultaneously maintain traffic paths with two base station apparatuses, and the choice between the two traffic paths can be made at a speech coding apparatus. When the mobile terminal forms a traffic path with a base station apparatus, the base station apparatus checks an information about traffic condition, that is, speech quality condition of the mobile terminal which forms the traffic path with the base station apparatus, and reports the information to the speech coding apparatus. Then, when a mobile terminal forms other traffic path in duplicate with other base station apparatus due to movement under a condition in which the mobile terminal forms a traffic path with an arbitrary base station apparatus and is engaged in a traffic, the speech coding apparatus analyses the speech quality condition information and selects as an actual traffic path the traffic path which is in better condition among the two traffic paths. Such change of traffic path is caused since the mobile terminal moves to a cell of other adjacent base station apparatus during the traffic in a base station cell, and such movement is called as "Hand-off" and becomes an important technical method for improving the speech quality in the CDMA mobile communication system.
Since the speech coding apparatus has to know where is the relevant pure speech information which the speech quality information points to(property of real time of speech quality information) at the time of decoding or analyzing the speech quality information to select a traffic path, the speech quality information uses an Inband Signalling method in which an inband signal is transmitted by forming a frame together with the pure speech quality information. Therefore, there are four types of information transmitted between the base station apparatus and the speech coding apparatus, that is, an information related to an call control and status control, the pure speech information, and the frame synchronization(sync) information and the inband signalling information which receives from the base station in real time a speech quality information of radio section when the mobile terminal is in traffic state. The pure speech information, frame sync information and inband signalling consists a packet frame to guarantee the property of real time of information and are transceived in a packet method
Since the speech coding apparatus shall receive from the base station the speech quality information of radio section in real time (that is, the speech quality information is needed at the same time with the relevant speech information) when a mobile subscriber is in traffic state in the conventional cellular mobile communication system which follows the CDMA method, the speech coding apparatus has to follow the inband signalling method and therefore uses the packet communication method which facilitates the constitution of frame and the transmission and reception. In this case, in the conventional cellular system, there are flexibility and conveniences in making an information frame and transmitting the information, however, the usage efficiency is low for transmission channel in view of the characteristics of packet method so that the subscriber accommodating capacity is reduced and there is a limitation on the constitution of apparatus being a cause of increase in constitution expenses. As an improvement the circuit communication method can be considered, however, since the packetized information must be transmitted in the circuit communication method in view of the CDMA method, the acquisition of frame sync between the base station and control station appears to be a difficult problem.